JP3681097B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus provided with a laser scanning device.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, a laser beam is scanned on a photoconductor, and the intensity of the laser beam and the lighting time are modulated for each pixel according to the value of an input signal. An electrostatic latent image is formed. The electrostatic latent image on the photosensitive member is developed by a developing device and transferred, fixed, or developed onto a recording medium, transferred to an intermediate transfer member, and then transferred to a recording medium, followed by an electrophotographic process for final output. It becomes an image.
[0003]
The semiconductor laser has a function of detecting the light emission intensity of the laser by a photodiode built in the unit and outputting a current corresponding thereto. APC (Automatic Power Control) is generally used in which this is fed back to a laser control circuit to control the emission intensity to always have a predetermined value. An example of the operation of APC will be briefly described with reference to FIG.
[0004]
The current flowing through the semiconductor laser 201 is roughly divided into two types: a driving current by controlling the transistor 203 and a bias current by controlling the transistor 204. The bias current is set to a value immediately before the semiconductor laser 201 emits laser light, that is, near the threshold current, and always flows to the semiconductor laser 201. This bias current is controlled so that the emission intensity of the laser becomes constant. The drive current is set to a fixed value such that the semiconductor laser 201 emits laser light. At the time of image formation, a pulse signal corresponding to the image is applied to the base of the transistor 203, and the current value determined by the pulse peak value and the resistor 205 becomes the drive current.
[0005]
The photodiode 202 is provided in the same unit as the semiconductor laser 201 so that a part of the laser beam of the semiconductor laser 201 is incident thereon. The photodiode 202 on which the laser light is incident outputs a current corresponding to the intensity of the laser light. The current-voltage converter 207 converts the output current of the photodiode 202 into a voltage signal with a predetermined amplification factor. The switch 208 is controlled by a control unit (not shown), and is turned on during APC control with a fixed period during image formation. When the switch 208 is turned on, the hold capacitor 209 accumulates charges so as to be equal to the output voltage of the current-voltage converter 207. The adder 210 outputs a voltage value obtained by subtracting the hold voltage value V ₁ of the hold capacitor 209 from the predetermined voltage value V ₀ . The output of the adder is applied to the base of the transistor 204 via the buffer amplifier 211, and the voltage value and the current value determined by the resistor 206 become the bias current.
[0006]
From the above, the emission intensity of the semiconductor laser 201 is determined by the total value of the drive current and the bias current. In the method of FIG. 2, since the drive current is a fixed value, fluctuations in the intensity of the laser beam due to temperature changes and changes with time are compensated by controlling the bias current.
[0007]
Next, the operation of APC will be described.
[0008]
During image formation, the semiconductor laser 201 is driven by a laser drive signal 301 in FIG. Here, when the BD is turned on, the laser is kept on for a predetermined time in the non-image area in order to generate a horizontal synchronization signal that is a reference for writing the image signal in the horizontal direction. In synchronization with this BD lighting, the control unit turns on the switch 208 by the APC mode signal 302. The photodiode 202 receives the laser beam of the semiconductor laser 201 when the BD is turned on, and outputs a current value corresponding to the laser beam. The output converted into a voltage signal by the current-voltage converter 207 is accumulated as a charge in the hold capacitor 209 via the switch 208 in the on state, and becomes equal to the output voltage of the current-voltage converter 207 during BD lighting. The switch 208 is opened again at a time earlier than the end of the BD lighting by a predetermined time, but the output voltage of the current-voltage converter 207 is held by the hold capacitor 209. A predetermined voltage V ₀ is input to one of the two inputs of the adder 210. The voltage value V ₁ held by the hold capacitor 209 is input to the other side. (V ₀ −V ₁ ) is output from the output of the adder 210. The bias current is determined by this (V ₀ −V ₁ ). That is, when the emission intensity of the semiconductor laser 201 is smaller than normal, V ₁ is small, (V ₀ −V ₁ ) is large, and the bias current is increased. On the other hand, when the emission intensity is higher than normal, V ₁ is large, (V ₀ −V ₁ ) is large, and the bias current is reduced.
[0009]
However, due to the temperature characteristics of the light emission intensity detecting photodiode, the detection current varies depending on the ambient temperature of the laser unit even with the same light emission intensity. As a result, there is a problem in that the light emission intensity of the laser changes depending on the ambient temperature and the image is affected.
[0010]
Therefore, in order to solve the above problem, temperature adjustment may be performed with a heater or the like in order to always keep the semiconductor laser at a predetermined temperature. FIG. 1 shows a semiconductor laser mounted with a temperature adjustment mechanism. A unit 201 including a semiconductor laser and a photodiode is fixed to a disk-shaped base 12 through an electrode through a hole formed in the center of the base 12. The thermistor 14 for detecting the surface temperature of the case is fixed to the case of the semiconductor laser unit 201 so as to contact the case. A control unit 400 shown in FIG. 4 is connected to both ends of the thermistor 14. A disk-shaped surface heater 13 is fixed to the base 12 so as to contact the base 12. This is a position where the vicinity of the semiconductor laser unit 201 fixed to the base 12 is heated. Since the base 12 is made of a material with sufficiently good heat conduction, the case of the semiconductor laser unit 201 can be quickly heated by the heater 13. As shown in FIG. 4, the heater 13 is connected to a power source in the control unit 400, and the power source can be turned on / off by the control unit 400.
[0011]
That is, when the surface temperature of the semiconductor laser unit 201 detected by the thermistor 14 is lower than a predetermined value, the heater power is turned on, and when it is higher than the predetermined value, the heater power is turned off. The reference temperature for controlling the heater 13 is determined as follows, for example. Assuming that Δα (° C.) is the difference between the temperature at which the semiconductor laser unit 201 is mounted in the apparatus and the ambient temperature, the control temperature T is set to T = k + Δα (the maximum temperature k (° C.) of the operating temperature range of the apparatus. ℃)
Set to.
[0012]
k = 35 ° C., Δα = 5 (° C.)
In this case, the control temperature T of the semiconductor laser 201 is T = 35 + 5 ° C.
[0013]
FIG. 4 shows a heater control unit 400. 5 V is divided by the thermistor 14 and the resistor 405, and the divided voltage is compared with the reference voltage generated by the resistors 401 and 402 by the comparator 403. Since the resistance value of the thermistor 14 changes depending on the surface temperature of the semiconductor laser unit 201, the voltage divided by the thermistor 403 changes depending on the temperature of the semiconductor laser 201. The values of the resistors 401, 402, and 405 are set so that the reference voltage is the same as the voltage divided by the thermistor 14 and the resistor 405 when the semiconductor laser 201 reaches the control temperature T. When the temperature falls below the control temperature T, the comparator 403 turns on the transistor 404 and power is supplied to the heater 13. When the voltage becomes equal to or higher than the control voltage T, the comparator 403 turns off the transistor 404, and the power supply to the heater 13 is cut off to adjust the temperature.
[0014]
FIG. 5 is a graph showing an example of the temperature change of the output current when a laser beam having a light emission intensity with a photodiode built in the semiconductor laser unit is detected.
[0015]
When the operating temperature range of the apparatus is 10 ° C. to 35 ° C., the operating temperature range of the semiconductor laser is 15 ° C. to 40 ° C., assuming that the temperature rise at the mounting position of the semiconductor laser is 5 ° C. From the characteristics shown in FIG. 5, the output current fluctuation of the photodiode in this range has a maximum fluctuation of about 3% based on the measured value at 25 ° C. When the temperature adjustment control value of the heater is set to 40 ° C. and the allowable range is set to ± 5 ° C., the output current fluctuation of the photodiode can be suppressed to a maximum of about 1% even if the measurement value of 40 ° C. is used as a reference.
[0016]
[Problems to be solved by the invention]
Conventionally, laser temperature adjustment has been performed regardless of whether or not an image is being formed. Since the current path of the heater is arranged near the current path of the laser, induction by a relatively large current that flows when the heater is turned on and off may affect the laser drive current. Specifically, the disturbance of the laser driving current for this induction disturbs the optical waveform of the laser that determines the image quality. In other words, since the heater is repeatedly turned on and off during image formation, there is a possibility that the influence of the heater current has an effect on the image.
[0017]
Furthermore, temperature adjustment is not performed only for the laser, but temperature adjustment is performed for each base and lens barrel to which the heater is attached. For this reason, the heat capacity is increased, the response speed of heat is decreased, and as a result, the heater ON / OFF cycle becomes longer, which may cause a large ripple in the laser temperature. This is shown in FIG. When the control temperature is T, a large ripple component exists as shown in FIG. This ripple component causes fluctuations in the laser power and degrades the image quality.
[0018]
The present invention provides an image forming apparatus that can perform high-precision temperature adjustment with a small amount of ripple, eliminates the influence of heater on / off due to temperature adjustment on image formation, and enables image formation with a stable laser light quantity. For the purpose.
[0019]
[Means for Solving the Problems]
An image forming apparatus according to the present invention includes a laser unit including a laser light source that emits laser light by a driving current and a light amount sensor that monitors the light amount of the laser light, and a photoconductor on which an image is formed by irradiation with the laser light. A laser control means for modulating the laser light according to an input image signal, a laser light quantity correction means for controlling a current for driving the laser light source based on an output of the light quantity sensor, and the laser light. Scanning means for main-scanning the region including the photosensitive member, scanning start signal generating means for detecting the main-scanned laser beam to form a scanning start reference signal, and the temperature of the laser unit are set. In an image forming apparatus having temperature control means for controlling temperature by turning on or off energization of a heating element so as to achieve a target temperature The temperature control during the image forming operation after reaching a predetermined temperature lower than the target temperature is determined from the scanning start reference signal for each main scanning, and is a non-image area in one line of each main scanning. It is characterized by being performed only in.
[0021]
Furthermore, the image forming apparatus according to the present invention is characterized in that in the above image forming apparatus, the temperature control means includes a heating means.
[0022]
Furthermore, the image forming apparatus according to the present invention is characterized in that in the above image forming apparatus, the temperature control means includes a cooling means.
[0023]
Furthermore, the image forming apparatus according to the present invention is characterized in that, in the above-described image forming apparatus, the temperature control means always performs temperature control until the laser unit reaches a predetermined temperature lower than the target temperature.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 6 is an example of the configuration of the printer unit of the image forming apparatus according to the present invention, and FIG. 7 is a timing chart thereof. In FIG. 6, an image signal sent from an external device such as an image scanner or a computer (not shown) is sent to the image writing timing control circuit 101. The image writing timing control circuit 101 modulates and drives the semiconductor laser 201 in accordance with magenta (M), cyan (C), yellow (Y), and black (BK) image signals. The laser beam is reflected by the rotating polygon mirror 103, fθ corrected by the f-θ lens 104, and scanned on the photosensitive drum 105. Thus, an electrostatic latent image is formed on the photosensitive drum 105.
[0025]
The photosensitive drum 105 is rotated in the clockwise direction by a drum motor 115 via a gear belt 116, and the transfer drum 108 is rotated counterclockwise in synchronism with the photosensitive drum 105 because the photosensitive drum 105 is connected to a gear (not shown). Driven in the direction of the arrow (side running). The drum motor 115 that rotationally drives the photosensitive drum 105 is rotationally driven by dividing the clock of the oscillator 112 by the frequency dividing circuit 117 and sending it to the PLL circuit 118 as a motor driving pulse (reference CLK). The PLL circuit 117 detects the phase difference and frequency deviation between the FG pulse and the reference CLK so that the motor FG pulse from the drum motor 115 and the reference CLK are in phase, and compares them to determine the drive voltage to the drum motor 115. Perform PLL control.
[0026]
The polygon motor 106 is driven to rotate by dividing the clock of the oscillator 112 by the frequency dividing circuit 113 and sending it to the PLL circuit 114 as a motor driving pulse (reference CLK). The PLL circuit 114 detects the phase difference and frequency deviation between the FG pulse and the reference CLK so that the motor FG pulse from the polygon motor and the reference CLK are in phase, and compares them to control the drive voltage to the polygon motor 106. PLL control is performed. The BD sensor 107 is provided in the vicinity of the scanning start position of one line of the laser beam, detects the line scanning of the laser beam, and generates a scanning start reference signal (BD signal) for each line having the same cycle as shown in FIG. produce. Around the photosensitive member 105, magenta (M), cyan (C), yellow (Y), and black (BK) developing units (not shown) are provided, and four developing units are rotated while the photosensitive drum 105 rotates four times. Are alternately in contact with the photosensitive drum 105 and developed with toner corresponding to the electrostatic latent images of M, C, Y, and BK formed on the photosensitive drum 105. A recording sheet 109 fed from a sheet cassette (not shown) is wound around a transfer drum 108, and a toner image developed by a developing device is transferred. In the transfer drum 108, there is a sensor 110 for generating an ITOP signal indicating the leading end position of the recording paper 109 on the transfer drum 108, and a flag 111 fixed in the transfer drum 108 by rotating the transfer drum 108 is a sensor 110. As shown in FIG. 7A, an ITOP signal for each color is created. After the four colors M, C, Y, and BK are sequentially transferred in this way, the sheet passes through a fixing unit (not shown) and is discharged.
[0027]
Since the temperature adjustment mechanism of the semiconductor laser is the same as that of FIG. 1 of the conventional example, the description thereof is omitted and the same reference numerals are also used in this embodiment. In this embodiment, the three-stage temperature adjustment method is switched depending on the temperature of the semiconductor laser. That is, assuming that the temperature adjustment set temperature is T, the heater is always turned on until a temperature T ′ obtained by subtracting a value slightly larger than the temperature ripple when the semiconductor laser unit 201 reaches T from the temperature adjustment temperature T. When the temperature becomes equal to or higher than the temperature T ′, the heater is turned on in a predetermined logic H section that can be set in synchronization with the BD lighting until the temperature T is reached. After reaching the temperature T, temperature adjustment is performed in a predetermined logic H section that can be set in synchronization with the BD lighting, such as heating off when the temperature is equal to or higher than T, and heating on when the temperature is equal to or lower than T. As a result, the temperature of the unit 201 reaches the temperature T ′ with a steep slope until the temperature T ′ as shown in FIG. 11, and then reaches the temperature T with a gentle slope from the time when the temperature reaches the temperature T ′. And then kept near the temperature T with a small ripple. Note that the predetermined logic H section that can be set in synchronization with the BD lighting is set to a non-image portion of main scanning, thereby eliminating the influence of heater on / off on image formation. That is, as shown in the timing chart of FIG. 10, the logic H is output only in the non-image areas such as A and B among the image areas / non-image areas with the maximum recording paper size determined in synchronization with the BD lighting. By enabling the temperature adjustment, it is possible to eliminate the influence of heater on / off on image formation. Since the temperature of the laser does not reach the set temperature T, there is no problem if the recording paper is small even if this temperature adjustment section (logic H section) enters the image area as in C, and both ends of the normal image area are blank areas. It is difficult to become a problem.
[0028]
FIG. 8 shows a temperature adjustment control unit of the semiconductor laser of this embodiment. First, control up to the temperature T ′ will be described. Since the resistance value of the thermistor 14 varies depending on the surface temperature of the unit 201 of the semiconductor laser, the voltage divided by the thermistor 14 and the resistor 814 varies depending on the temperature of the semiconductor laser 201. The values of the resistors 814, 816, and 817 are the thermistor 14 when the temperature of the semiconductor laser unit 201 becomes a temperature T 'minus a value slightly larger than the temperature ripple when the temperature adjustment temperature T reaches T. The voltage divided by the resistor 814 and the reference voltage are set to be the same. Since the comparator 815 outputs the logic H to the OR gate 808 until the temperature T ′, the logic H is input to one input of the AND gate 807. The values of the resistors 811 and 812 are set so that the reference voltage is the same as the voltage divided by the thermistor 14 and the resistor 814 when the semiconductor laser 201 reaches the temperature adjustment temperature T. Therefore, the comparator 813 continues to output the logic H to the AND gate 807 until the temperature adjustment temperature T is reached. Therefore, until the temperature T ', the AND gate 807 is input with logic H at both inputs, so that the transistor 814 is always turned on and power is supplied to the heater.
[0029]
Next, since the output of the comparator 815 is logic L from the temperature T ′ to T, the output of the OR gate 808 outputs only the output of the inverter 806 to the AND gate 807. Since the output of the comparator 813 is always logic H up to the temperature T, the transistor 814 is turned on and power is supplied to the heater only when the output of the inverter 806 that outputs a settable predetermined logic H section synchronized with BD lighting is logic H. Is done. An output in a predetermined logic H section that can be set in synchronization with the BD lighting is created as follows. In other words, the counter 801 counts the clock from the crystal oscillator 802, and at the time of image formation, the main scanning start reference signal (BD) from the BD sensor 107 selected by the selector 820 by the image formation signal from the control unit (not shown). Reset. Since the BD signal is not output except for image formation, it is reset by a ripple carry from the counter 821 that counts the clock output from the oscillator 802. The counter 821 is preset so that a ripple carry is generated at substantially the same cycle as the BD cycle. The value of the counter 801 is input to the comparators 803 and 804. The comparators 803 and 804 receive a value determined according to the region in the main scanning direction permitting temperature adjustment by a control unit (not shown). The comparator 803 receives a set value A that represents the start point for prohibiting main scanning temperature adjustment, and the comparator 804 receives the set value B that represents the start point for permitting main scanning temperature adjustment. The JK-FF 805 is set by the comparator 803 when the set value A and the value of the counter 801 are the same, and is reset by the comparator 804 when the set value B and the value of the counter 801 are the same. The logic H is maintained in the temperature adjustment prohibition region in FIG. The inverter 806 inverts the signal from the JK FF 805 and outputs a logic H to the OR gate 808 in the temperature adjustable region in the main scan. In this way, a settable predetermined logic H section synchronized with BD lighting is created. It is obvious that the position where the logic H is output and the DUTY can be varied by changing the values set in the comparators 803 and 804.
[0030]
Once the control temperature T is reached and becomes equal to or higher than the control temperature T, the comparator 813 outputs a logic L to the AND gate 807, and the heater 13 is turned off. When the heater 13 is turned off, the temperature is adjusted such that the heater is turned on in a predetermined logic H section that can be set in synchronization with the BD lighting when the temperature becomes T or less. Note that the resistor 818 and the transistor 819 are provided for providing hysteresis to the output of the comparator 813. That is, when the output of the comparator becomes logic H, the transistor 819 is turned on, so that the reference voltage input to the comparator is lowered. When the output of the comparator 813 becomes logic L, the transistor 819 is turned off to return to the original reference voltage. That is, the hysteresis is provided by changing the reference voltage when the output of the comparator 813 changes from logic L to H and when the output changes from logic H to L.
[0031]
FIG. 9 is a timing chart of a settable predetermined logical H section synchronized with the BD lighting at the time of image formation. When the BD signal rises, the counter 801 counts the clock, and when it becomes the same as the set value A indicating the start point where the scanning temperature adjustment is prohibited, the comparator 803 outputs a pulse and starts allowing the main scanning temperature adjustment. When the set value B representing the point becomes the same, the comparator 804 outputs a pulse. Since the JK-FF 805 is set and reset by this output, a logic H is output during this time. Since the output of the JK-FF is inverted by the inverter 806, the heater can be turned on only in the temperature adjustment permission region.
[0032]
In the above embodiment, the temperature adjustment is performed using the heater. However, a cooling unit using the Peltier effect can be used instead of the heater. In this case, the control temperature is set near the lower limit of the temperature range of the semiconductor laser case 201, for example.
[0033]
In this way, once the temperature adjustment point is reached, only a part of the main scanning line is temperature-adjusted, so the heater can be turned on and off in a fast cycle. As a result, fine temperature adjustment, that is, there is little ripple Temperature adjustment is possible.
[0034]
Furthermore, by making the non-image area of main scanning mainly the temperature adjustable area, it is possible to eliminate the influence on the laser control due to the heater on / off, and it is possible to form an image with a stable laser light quantity.
[0035]
【The invention's effect】
As described above, by adjusting the temperature of the semiconductor laser at the time of image formation in synchronization with the main scanning start signal, it is possible to perform high-precision temperature adjustment with a small amount of ripple, and the effect of temperature adjustment on the image formation of the heater on / off It is possible to eliminate and to form an image with a stable laser light quantity.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a laser temperature adjustment mechanism in the present invention and a conventional example.
FIG. 2 is a circuit diagram of a circuit for performing APC of a laser in the present invention and a conventional example.
FIG. 3 is a timing chart showing timing for performing APC of a laser in the present invention and a conventional example.
FIG. 4 is a circuit diagram of a laser temperature adjustment control unit according to a conventional example.
FIG. 5 is a graph showing temperature characteristics of a photodiode built in a laser according to the present invention and a conventional example.
FIG. 6 is a conceptual diagram illustrating an image forming apparatus according to an embodiment of the present invention and a conventional example.
FIG. 7 is a timing chart showing the timing of BD and ITO in the present embodiment of the present invention and a conventional example.
FIG. 8 is a circuit diagram of a laser temperature adjustment control unit according to an embodiment of the present invention.
FIG. 9 is a timing diagram showing an operation of the laser temperature adjustment control unit according to the embodiment of the present invention.
FIG. 10 is a timing diagram illustrating a laser temperature adjustable region according to an embodiment of the present invention.
FIG. 11 is a graph showing laser temperature transition by laser temperature adjustment according to an embodiment of the present invention.
FIG. 12 is a graph showing laser temperature transition by laser temperature adjustment according to a conventional example.
[Explanation of symbols]
12 Base 13 Heater 14 Thermistor 15 Heater electrode 16 Collimator lens 17 Lens tube 107 BD sensor 201 Semiconductor laser unit 801, 821 Counter 803, 804 Comparator 813, 815 Comparator 820 Selector

Claims (4)

A laser unit comprising a laser light source that emits laser light by a driving current and a light amount sensor that monitors the light amount of the laser light;
  A photoreceptor on which an image is formed by irradiation with the laser beam;
  Laser control means for modulating the laser beam in accordance with an input image signal;
  A laser light amount correcting means for controlling a current for driving the laser light source based on an output of the light amount sensor;
  Scanning means for main-scanning a region including the laser beam on the photosensitive member;
  Scanning start signal generating means for detecting the laser beam that has undergone main scanning and forming a scanning start reference signal;
  Temperature control means for performing temperature control by turning on or off energization of the heating element so that the temperature of the laser unit becomes a set target temperature;
  In an image forming apparatus having
  The temperature control during the image forming operation after reaching a predetermined temperature lower than the target temperature is performed only in a non-image area in one main scanning line determined from the scanning start reference signal for each main scanning. An image forming apparatus. The image forming apparatus according to claim 1, wherein the temperature control unit includes a heating unit. The image forming apparatus according to claim 1, wherein the temperature control unit includes a cooling unit. It said temperature control means image forming apparatus according to any one of claims 1 to 3, characterized in that at all times the temperature control until said laser unit reaches the targets temperature lower than the predetermined temperature.

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